This invention relates to digital multimeters capable of performing multiple functions such as voltage, resistance and current measurement, and more particularly to a two-terminal digital meter that provides microamp measurement capabilities.
Multimeters in general and digital multimeters (DMMs) in particular, employ a common and a volt/ohms input terminal. Meters that provide microamp current measurement employ a third input terminal that is separate from the volt/ohms input terminal. Therefore, is to measure microamps, a meter user must pull the test probe out of the volt/ohms terminal and insert it into the microamps terminal, before taking the measurement.
Especially with measuring microamps, it is desirable to provide protection circuitry, to avoid damaging a meter in case the user accidentally touches the probe tips to the wrong place and overloads the meter. Such protection typically comprises a fuse that is chosen to blow before the rest of the meter circuitry can be damaged. However, having to change fuses is annoying, especially when it results from an accidental slip of a test probe into an unintended test point.
Referring to FIG. 1, which is a diagram of input circuitry of a typical prior art digital multimeter, the meter 10 comprises a common input terminal 12, a volts/ohms input terminal 14, and a microamp input terminal 16. A shunt resistor 20 is placed between the common and microamp input terminals, with a fuse (F1) 18 in line with the microamp input terminal. The fuse is chosen to blow if the current exceeds a predetermined amount. Further provided across the microamp and common terminals is diode bridge circuitry 22, which provides further input protection. A protection resistor (100 k ohm in the illustration) is used to limit current into a single chip analog to digital (A-D) converter 26 when there is an overload. The measurement system 26, which ,as noted, suitably comprises a single chip. analog to digital (A-D) converter, receives the output of resistor 24 and passes it through a switch 28, whereupon an A-D converter circuit is provided the input signal for measurement. Switch 28 suitably comprises a field effect transistor (FET) switch.
Other details of the prior art system include a switch 30 and resistor 32 in series in the common input line, the output thereof being provided to FET switch 34, which then is supplied to the A-D converter. The volts/ohms input terminal has a series resistor 36. which supplies the input signal to FET switch 38, the output thereof being connected to the junction of resistor 32 and FET switch 34. The various switches are actuated in accordance with the measurement to be taken. However, the multimeter user must physically move the input probe from the volts/ohms input terminal to the microamp input terminal in order to perform a microamp measurement. Once the current measurements are completed, then the probe must be moved back to the volts/ohms terminal if non-current type measurements are to be taken. Another drawback to the prior art system is that if the wrong input terminals are selected for a particular measurement, the multimeter can be damaged.
In accordance with the invention, a two-terminal multimeter is provided that enables voltage/ohm/microamp measurements without need of a third terminal. A first input and a common input are provided. A reference resistor is provided at either side thereof with a switch to ground, and these switches are activated to either ground the first or second end of the resistor. The first and second ends are also suitably connected to plus and minus sides of an A-D converter for obtaining appropriate measurement it values.
Accordingly, it is an object of the present invention to provide an improved digital multimeter that enables microamp measurements without need of a third input terminal.
It is a further object of the present invention to provide an improved multimeter that employs existing input protection and measurement components within a meter to enable microamp or volt/ohm measurements via only two terminals.
It is yet another object of the present invention to provide an improved system for volt/ohm/microamp measurement in a two terminal input meter.
A further object of the present invention is to provide an improved meter that is capable of measuring DC or AC microamps in addition to volts/ohms or other typical multimeter measurement functions with a two terminal input.
The subject matter of the present invention is particularly pointed out and distinctly claimed in the concluding portion of this specification. However, both the organization and method of operation, together with further advantages and objects thereof, may best be understood by reference to the following description taken in connection with accompanying drawings wherein like reference characters refer to like elements.